Biological Effects of Ionising Radiations 135 



neutron radiation were needed to produce a given degree of 

 skin reaction, and one may say roughly that the efficiency of 

 neutrons in this respect appears about 2.5 times as great as 

 X-rays. It is important to emphasize, however, that, as in X-ray 

 therapy, the total dose was delivered in a large number of frac- 

 tions spread over about 3 weeks, and until the influence of 

 fractionation on the effects of both types of radiation has been 

 fully investigated, a figure representing their apparent relative 

 efl^ectiveness gives little guide as to the nature of the processes 

 involved.*^ It is at least clear, however, that both skin response 

 and tumor response belong to the class of reactions in which, 

 proceeding from gamma rays to neutrons, the efifectiveness in- 

 creases with increasing ion density. It has been pointed out ^^ 

 that, insofar as more favorable tumor response has been obtained 

 with neutrons than with X-rays, this may be taken to indicate 

 that the curve (figure 3) for tumor response is rising more 

 rapidly than that for skin-damage. A further improvement 

 might therefore be expected by the use of less energetic (greater 

 ion density) neutrons, and advantage might be taken of the fall 

 in the average energy of a neutron beam on passing into the 

 body to increase the damage to the tumor relative to that to 

 the skin. 



Such an advantage, however, falls into the same class as the 

 technical improvement offered by the increased depth-dose 

 obtained with high-voltage X-ray tubes and betatrons. At best, 

 they enable the therapist to deliver any desired dose of radiation 

 to a mass of tissue which completely envelops all the malignant 

 cells. There remains the problem of discriminating between 

 two adjacent cells in such a manner as to destroy either the 

 malignant character of the tumor cell or the cell itself, without 

 destroying all its healthy neighbors. Such discriminations must 

 be based ultimately on a biological difference between the two 

 cells. Differences in matabolism, chromosome structure, and 

 rate of development, are known to exist, and these differences, 

 as we have seen, profoundly affect the manner in which the 

 various functions of a cell are influenced by radiations of differ- 

 ing ion density. It would seem that a fuller investigation of 



